Often there are storage facilities where a large number of assets are stored. In order to track the assets, a radio frequency identification (RFID) tag is provided on each asset. Each tag includes a receiver that can receive wireless signals, and a transmitter that can transmit wireless signals. The storage facility has at least one stationary reader with a transmitter and receiver. Each reader can engage in two-way communications with a number of tags using radio frequency (RF) wireless signals. In this regard, a reader in this type of environment typically does periodic polling of the tags within its transmission range, in order to see if any new tags/assets have arrived, and in order to see if any tags/assets that were previously present have departed. This permits automated inventory control.
One common technique for polling tags is to have a reader transmit a wireless “wakeup” signal every 30 seconds. The wakeup signal asks that nearby tags each transmit a wireless reply in order to identify themselves. The tags operate on battery power. To conserve battery power, tags typically have multiple operating modes, including a normal operational mode and also a low power “sleep” or “rest” mode, in which most but not all of the tag's circuitry is powered down in order to reduce battery drain. The tags remain in the low power sleep mode most of the time, but every few seconds they each switch to the normal operational mode for a few milliseconds, in order to check for the presence of a wakeup signal. If no wakeup signal is detected, the tag returns to its low power sleep mode. But if a wakeup signal is detected, the tag remains in the normal operational mode a little longer, in order to receive the entire signal from the reader, and then transmit a reply back to the reader.
Even though the tags are in the low power sleep mode most of the time, the small amounts of time spent in the normal operational mode add up over a long period of time. And where a tag remains in the same location for a long period of time, the tag spends a significant amount of time and battery power receiving and responding to numerous wakeup signals, without telling the reader much beyond what the tag previously told the reader in earlier communications. This is referred to herein as “overpolling”, and involves unnecessary power drain on the batteries of the tags.